I've been shopping with Gamestop for quite a while now and never had problems with them such as the ones you've all listed. Every game I've bought there was factory-sealed.
BUT, hearing all this definitely makes me more cautious. Just reading about crap like that is making me angry. I don't know how I'd walk out from a bullshit situation like that without running my hand across their row of games, knocking them down as I walk out screaming obscenities.
Same here. Gamestops/EBs are only as good as the people who work there. I always hated them, until I found the one I am currently using. The guys there are nice, will go out of their way to give you what you want. If you bring the one of the open "new" games, they will set it aside and get an unopened game for you.